August 2002 City of Wanneroo Quinns Beach Erosion ... · M P ROGERS & ASSOCIATES Quinns Beach...

August 2002

City of Wanneroo

Quinns Beach Erosion & Coastal Protection

M P ROGERS & ASSOCIATES

Coastal & Port Engineers

Job J248/7 Report R110 Rev 1

August 2002

City of Wanneroo

Quinns Beach Erosion & Coastal Protection

Job J248/7 Report R110 Rev 1 - Record of Document Revisions

Rev Purpose of Document Prepared Reviewed Approved Date

A Draft for MRA & Client review M Rogers S Gerbaz M Rogers 28/6/02

0 Minor revisions & issued for Client use M Rogers S Gerbaz M Rogers 23/7/02

1 Minor revisions & issued for Client use M Rogers S Gerbaz M Rogers 12/8/02

Form 035 27/8/00

M P ROGERS & ASSOCIATES - Coastal & Port EngineersUnit 2, 133 Main Street, Osborne Park WA 6017, AUSTRALIA

Phone: +618 9444 4045, Fax: +618 9444 4341Email: [email protected]

M P ROGERS & ASSOCIATES Quinns Beach Erosion & Coastal ProtectionJob J248/7 Report R110 Rev 1, Page (i)

Limitations of this ReportThis report has been prepared for use by the Client in accordance with theagreement between the Client and M P Rogers & Associates Pty Ltd. Thisagreement includes constraints on the scope, budget and time available forthe services. The consulting services and this report have been completedwith the degree of skill, care and diligence normally exercised by membersof the engineering profession performing services of a similar nature. Noother warranty, expressed or implied, is made as to the accuracy of the dataand professional advice included in this report. This report has not beenprepared for use by parties other than the Client and its consulting advisers.It may not contain sufficient information for the purposes of other parties orfor other uses.

M P Rogers & Associates takes no responsibility for the completeness orform of any subsequent copies of this report. Copying this report withoutthe permission of the Client or M P Rogers & Associates Pty Ltd is notpermitted.

M P ROGERS & ASSOCIATES Quinns Beach Erosion & Coastal ProtectionJob J248/7 Report R110 Rev 1, Page (ii)

Table of Contents1. Executive Summary 12. Introduction & Background 53. Modelling the Coastal Processes 84. Option 1 – Groyne Field & Initial Beach

Nourishment 125. Option 2 – Headlands & Initial Nourishment 186. Option 3 – Seawall & Ongoing Nourishment 227. Comparison of Options 258. References 279. Figures 2810. Appendices 38

M P ROGERS & ASSOCIATES Quinns Beach Erosion & Coastal ProtectionJob J248/7 Report R110 Rev 1, Page 1

1. Executive SummaryThere has been a long history of coastal erosion at Quinns Rock. In the1970s, erosion of the southern flank was stopped by the construction of arock seawall near the toilet block near the end of Quinns Road and a smallrock headland built near the Quinns Cusp in 1977. The later work movedthe erosion from the Southern Beach to the Northern Beach. Between 1977and 1997 the Northern Beach retreated about 20 metres due to the coastalerosion.

The data suggests that the recession of the Northern Beach was causedlargely by the construction of the headland in 1977. The recession does notseem to have been affected by the construction of the breakwaters as part ofthe Mindarie Keys project. The rate of recession of the Northern Beach wassimilar for the period before and after the construction of Mindarie Keys.

Coastal erosion in the late 1990s led to coastal and port engineers M PRogers & Associates being engaged to study the situation. The datasuggested that the historical rate of erosion of the Northern Beach was about8,500 m3/year and the Southern Beach had been accreting at about 4,000m3/year. Based on the data, it was estimated that the future rate of erosionof the Northern Beach would be about 7,000 m3/year. The small netmovement of sand to the north was thought to be the result of the differencein much larger seasonal movements of sand along the beach. It wasestimated that the seasonal movements along the Northern Beach could bein the order of 100,000 m3 to the north in summer and autumn and a similarbut slightly smaller flux to the south in winter and spring. The comparisonof options was completed using these estimates.

The Department for Planning & Infrastructure surveyed the beaches inJanuary 2000 and the survey data coupled with beach nourishment recordsindicated that the rate of erosion between December 1997 and January 2000was about 6,000 m3/year (insitu volume). This loss of sand was offset bythe nourishment works completed by the City of Wanneroo and the coastremained reasonably stable. The performance of the sand nourishment workwas not ideal in that some recession was still experienced at FrederickStubbs Reserve. This suggested that some increase to the quantity of sandand adjustment to the location of future nourishment exercises would bewarranted.

In late 2001 significant erosion of the Northern Beach was observed and theDepartment for Planning & Infrastructure completed a survey in February2002. The results of this survey indicated that about 78,000 m3 of sand hadbeen lost over the 2 years between the 2000 and 2002 surveys. Othermetropolitan beaches were also suffering from erosion at this time. It was


thought that perhaps the fact that the winter of 2001 was particularly mildwith fewer than usual storms could be a contribution. The winter storms areimportant at Quinns in that they can move sand from the beaches further tothe north back to the beaches at Quinns.

The occurrence of such unusual weather conditions was not identified orconsidered in the assessment of options in Rogers & Associates (1999). Theoccurrence of such unusual weather may occur again, and the effectivenessof the Renourishment and Seawall option and the Groyne option need to beinvestigated under these conditions.

This study was commissioned by the City of Wanneroo with the agreementof the Department for Planning and Infrastructure to investigate:

• the reasons for the rapid erosion between January 2000 and February2002 and revise the assessment of coastal processes accordingly,

• reassess the effectiveness and costs of the Renourishment and Seawalloption for the revised assessment of coastal processes,

• reassess the effectiveness and costs of the Groyne option for the revisedassessment of coastal processes, and

• reassess the effectiveness and costs of using headlands instead of groynesfor the revised assessment of coastal processes.

This work included the set-up and calibration of a computer model of themovement of sand along the coast under the influence of waves. The modelwas run for the usual wave conditions and calibrated to match the observedlong-term changes. Then the wave conditions experienced in 2001 wereused as input to the model.

In 2001 there were only about half the usual number of winter storms. Themodel indicated that the lower number of winter storms had a large effect inthat much less a quantity of sand was moved from the north into the beachesat Quinns than usual during winter. The quantity of erosion calculated usingthe calibrated computer model was similar to that measured in the DPIsurveys. The computer modelling indicated that the rapid erosion in 2001was largely due to the lack of winter storms.

The computer model was then used to assess the performance of the threeoptions above. The modelling indicated that all three options could be usedto deal with the erosion at Quinns. The use of groynes or headlands to alterthe longshore transport regime could be effective but would not totallyisolate the beaches from the impacts of very mild winters such as


experienced in 2001. There would need to be some additional beachnourishment following such events. The extent of the beach nourishmentwould be less for the options using the groynes or headlands compared tothe seawall and ongoing sand nourishment.

The Net Present Value of costs for the three options evaluated in this reportare presented in Table 1.1 below. This table also lists the Capital Cost ofthe three options.

Table 1.1 NPV & Capital Costs for Options 1, 2 and 3

Option NPV @ 4% NPV @ 6% Capital Cost

Option 1 – Groynes &Initial Nourishment

$3,738,000 $3,455,000 $2,711,000

Option 2 – Headlands &Initial Nourishment

$3,880,000 $3,585,000 $2,807,000

Option 3 – Seawall &Ongoing Nourishment

$3,937,000 $3,315,000 $1,366,000

Note: 1. Refer to Appendix A for details of discounted cash flow analysis.

This comparison indicates that all three options have quite similar NetPresent Value of costs over a 35 year period using a 4% or 6% discount rate.Certainly, the differences are less than the likely error bands in the variouscost estimates.

Option 1 has the lowest NPV when using a 4% discount rate. In addition,this option involves significant capital expenditure that can be sharedbetween the City of Wanneroo and the State Government and does not relyso much on recurrent expenditure funding.

Option 3, by comparison has the lowest NPV when using a 6% discountrate. It involves about half the amount of capital expenditure and is muchmore reliant on recurrent expenditure.

The significant ongoing sand nourishment works in Option 3 may not beacceptable to the local residents after a number of years. Manycommunities have the belief that ongoing beach nourishment is wastingmoney despite the known benefits. Consequently, because of possiblechanges to community attitudes, management and government policy, thelong term funding of significant ongoing recurrent expenditure such as inOption 3 may not be secure. If the recurrent funding were not available infuture years, then the performance of the scheme would be compromised.


Option 1 uses groynes to adjust the longshore transport regime and movethe erosion problem further along the coast. The use of groynes providesthe opportunity to make minor adjustments to the scheme should there bedifferences between the computer model predictions and the scheme asbuilt. This is important because the real coastal processes are very complexand the interaction of a number of factors. The modelling work is asimplification of the actual system.

Option 1 is recommended to be adopted for the long term solution to theerosion at Quinns because it has one of the lowest NPV of costs andprovides a practical scheme that can be staged over a 3 to 4 year period. Italso provides some scope for fine-tuning, should it be required.

It is recommended that the City of Wanneroo and the State Governmentreview this report, discuss the options and jointly adopt a preferred option.Then the recent erosion, this report and the preferred option should bepresented to a public meeting. At this public meeting, the acceptability ofthe use of groynes can be discussed, and hopefully the preferred option putforward by the City of Wanneroo and the State Government will besupported.

Following the public meeting, the detailed design and documentation of thepreferred option will need to be completed. The work should then be put topublic tender and constructed at the end of the coming spring to takeadvantage of the movement of sand to the south in winter and spring.


2. Introduction & Background2.1 BackgroundThere have been coastal erosion problems at Quinns for many decades. In1970 the Shire of Wanneroo constructed a seawall to protect the parking lotand toilet block located at the end of Quinns Road (refer to Figure 2.1).Additional protection works were completed in 1977 when a rubbleheadland was built near the Quinns Cusp to encourage accretion along theSouthern Beach. Following severe storms in 1996, sand nourishment wasplaced on the Southern Beach to protect Ocean Drive and on the NorthernBeach to protect the Northern Car Park.

In 1997, Tremarfon Pty Ltd investigated the erosion and concluded that theerosion on both the Southern and Northern Beaches may be a result of thestormy winter in 1996. It was hoped that the situation would recover andsand nourishment was suggested for a 5 year period. The City of Wanneroohas been completing sand nourishment each year since receipt of theTremarfon report.

In 1998 the City of Wanneroo engaged M P Rogers & Associates Pty Ltd toinvestigate the erosion and develop the best long term solution. Rogers &Associates (1998) indicated that the construction of the headland in 1977helped halt the erosion of the Southern Beach but has caused the NorthernBeach to erode. Between 1977 and 1997 the coastal vegetation line near theNorthern Car Park and Toilet Block has retreated about 20 metres. Theavailable information suggested that the recession was reasonablyprogressive although there were periods of more rapid recession. Thisfinding was quite different to that in Tremarfon (1997). Differences in theassessment of coastal processes can occur because of the complexity of thenatural systems and the very limited data available for the analysis.

The information suggested that the recession of the Northern Beach wascaused by the construction of the headland in 1977. The recession does notseem to have been affected by the construction of the breakwaters atMindarie Keys. The rate of recession was similar for the period before andafter the construction of Mindarie Keys.

2.2 Beach Erosion 1997 to 2002In Rogers & Associates (1999) the historical rate of erosion from theNorthern Beach had been estimated to be about 8,500 m3/year (insituvolume). The Southern Beach was accreting at an average rate of 4,000m3/year between 1977 and 1997. These estimates were based on the limitedsurvey data augmented by shoreline movement plans and review ofhistorical aerial photographs. Because the Southern Beach appeared to be


stabilised, the future rate of erosion of the Northern Beach had beenestimated to be about 7,000 m3/year (insitu volume). The small netmovement of sand to the north was thought to be the result of much largerseasonal movements. It was estimated that the seasonal movements alongthe Northern Beach could be in the order of 100,000 m3 to the north insummer and autumn and a similar but slightly smaller flux to the south inwinter and spring. The comparison of options was completed using theseestimates.

The Department for Planning & Infrastructure surveyed the beaches inJanuary 2000 and the survey data coupled with beach nourishment recordsindicated that the rate of erosion between December 1997 and January 2000was about 6,000 m3/year (insitu volume). This loss of sand was offset bythe nourishment works completed by the City of Wanneroo and the coastremained reasonably stable. The performance of the sand nourishment workwas not ideal in that some recession was still experienced at FrederickStubbs Reserve. This suggested that some increase to the quantity of sandand adjustment to the location of future nourishment exercises would bewarranted.

In late 2001 significant erosion of the Northern Beach was observed and theDepartment for Planning & Infrastructure completed a survey in February2002. The results of this survey indicated that about 78,000 m3 of sand hadbeen lost over the 2 years between the 2000 and 2002 surveys. Othermetropolitan beaches were also suffering from erosion at this time. It wasthought that perhaps the fact that the winter of 2001 was particularly mildwith fewer than usual storms could be a contribution. The winter storms areimportant at Quinns in that they can move sand from the beaches further tothe north back to the beaches at Quinns.

The occurrence of such unusual weather conditions was not identified orconsidered in the assessment of options in Rogers & Associates (1999). Theoccurrence of such unusual weather may occur again, and the effectivenessof the Renourishment and Seawall option and the Groyne option need to beinvestigated under these conditions.

The City of Wanneroo has surveyed the position of the toe of the erosionbank along the Northern Beach. The surveys have been completed on 26April, 9 May and 13 May 2002. These surveys show that the recession hascontinued. Between 22 February and 13 May 2002, the erosion scarp nearthe Northern Car Park has moved landward about 10 to 15 metres. This issignificant and needs to be included in the analysis of options.


2.3 Funding EnvironmentRogers & Associates (1999) examined a range of options for the long termstabilisation of the Northern Beach. The analysis considered both capitaland recurrent costs over a 35 year period. The Net Present Value of costsfor the Renourishment and Seawall option and the Groyne option were quitesimilar. The Renourishment and Seawall option was assessed to have lessimpact on the aesthetics and beach use than the Groyne option. On thisbasis and because there was some opposition to groynes by some people inthe local community, the Renourishment and Seawall option was putforward as the preferred long term solution.

In the last few years, it has become apparent that there could be futurechanges in management, government and community attitudes which maymake funding large recurrent expenditure for coastal stabilisation works lesssecure in future years. This makes the implementation of options involvingongoing recurrent expenditure less secure than those involving expenditureprimarily in the first few years. In other words, the ongoing funding of theRenourishment and Seawall option may not be secure, regardless of thepresent day attitude of the parties providing the funding.

2.4 Scope of Present StudyThis study was commissioned by the City of Wanneroo with the agreementof the Department for Planning and Infrastructure to investigate:

• the reasons for the rapid erosion between January 2000 and February2002 and revise the assessment of coastal processes accordingly,

• reassess the effectiveness and costs of the Renourishment and Seawalloption for the revised assessment of coastal processes,

• reassess the effectiveness and costs of the Groyne option for the revisedassessment of coastal processes, and

• reassess the effectiveness and costs of using headlands instead of groynesfor the revised assessment of coastal processes.


3. Modelling the Coastal Processes3.1 Calibration / Validation of GENESIS ModelThe GENESIS model was developed by the US Army Corp of Engineers(1991). It is a generalised model for simulating shoreline change resultingfrom gradients in the movement of sand along the beach caused by waves.This model requires a number of parameters as input to the model. Thefollowing are the main input data.

• Sediment size,

• Shoreline alignment and location of structures such as headlands andgroynes,

• Depth of active zone and level of beach berm, and

• Nearshore wave conditions.

Estimates of most of this data were available from the earlier coastalengineering studies. The nearshore wave conditions for the following keywave conditions were available from computer modelling completed in theearlier work.

• Swell,

• Sea breeze and land breeze cycle, and

• Winter storm activity.

DPI operates a non-directional Waverider Buoy in 50 meters of watersouthwest of Rottnest Island. Review of the DPI wave records and winddata from the Bureau of Meteorology was completed to determine thefrequency of occurrence of the various wave events. This was done for theperiod 1995 to 1999, 2000 and 2001. The results are shown in Table 3.1below.


Table 3.1 Annual Number of Days of Key Wave Events

Wave Event Average1995 to 1999

2000 2001

Large Swell (days) 70 86 88

Light Swell / Calm (days) 111 97 94

Land / Seabreeze (days) 150 148 171

Usual Winter Storms (days) 30 34 18

Severe Winter Storms (days) 4 0 0

Notes: 1. The number of days of each wave event was manually estimated by review ofavailable wave and wind records.2. This analysis is approximate only.3. Swells greater than 2m are considered large.

This table highlights that the winter of 2001 was particularly mild with farless storm activity than usual. In addition, there was more seabreeze activityand large swell than usual. The lack of winter storm activity and increasedseabreeze and swell could greatly affect the transport of sediment along theshore at Quinns. The net sediment transport along the beach to the northcould be much larger than usual because the northwesterly and westerly seasin the winter storms usually moves a large volume of sand along the beachesfrom north to south.

The GENESIS model was setup for the undeveloped Northern Beach andrun using the usual number of wave events as per 1995 to 1999 in Table 3.1.The run was calibrated to obtain the gross and net sediment fluxes derivedfrom the analysis of surveys between 1977 and 1997. The model gaveseasonal fluxes of about 80,000 m3 and the net movement of about 7,000m3/year to the north. The model run indicated that the Southern Beachwould accrete at about 3,000 m3/year and the Northern Beach would erodeat about 10,000 m3/year. This is reasonable agreement with the surveyresults between 1977 and 1997 where Southern Beach was accreting atabout 4,000 m3/year and the Northern Beach was eroding at about 8,500m3/year. The GENESIS model was predicting a slightly higher rate oferosion of the Northern Beach but slightly lower seasonal fluxes andaccretion on the Southern Beach.

This GENESIS run indicates that the model can reasonably predict thelongshore movements and associated areas of accretion and erosion atQuinns. It was concluded that the GENESIS model provides an appropriate


tool for the representation of the dominant longshore coastal processes atQuinns. The model would be suitable to examine the recent episode oferosion in 2000 and 2001 and examination of various schemes to stabilisethe beaches at Quinns. Naturally the model is a simplification of thecomplex coastal processes at Quinns.

The anecdotal evidence of the erosion between the January 2000 andFebruary 2002 surveys suggests that most of the erosion occurred towardsthe end of 2001. The GENESIS model was then run using the number ofwave events estimated for 2001. This run gave a very different resultcompared to the typical wave conditions. The net transport was about74,000 m3 to the north for the year. About 42,000 m3 was lost from theSouthern Beach and 32,000 m3 lost from the Northern Beach. This isconsistent with the results of the analysis of the surveys from January 2000and February 2002 where the calculated loss was some 78,000 m3. Thesurveys also indicated that about 43,000 m3 was lost from the SouthernBeach and 35,000 m3 was lost from the Northern Beach.

A summary of the survey and GENESIS results for various periods andlocations are presented in Table 3.2 below.

Table 3.2 GENESIS Results Compared to Survey Data

Item Surveys GENESIS

Gain in Southern Beach for average year 4,000 m3/yr 3,000 m3/yr

Loss from Northern Beach for average year 8,500 m3/yr 10,000 m3/yr

Seasonal fluxes for average year 100,000 m3/yr 80,000 m3/yr

Loss in Southern Beach for 2001 conditions 43,000 m3/yr 42,000 m3/yr

Loss in Northern Beach for 2001 conditions 35,000 m3/yr 32,000 m3/yr

The model runs provide some confirmation and confidence that the cause ofthe large recent erosion was the mild winter of 2001 coupled with higherthan average seabreeze and swell activity.

There is very limited data on the frequency of occurrence of conditions suchas experienced in 2001. There are wave and wind records for the 7 yearssince 1995. On the face of it, the mild winter conditions could occur onceevery 7 years. Figure 3.1 shows the position of the coastal vegetation line atthe Northern Beach Car Park between 1977 and 1998. It appears that thereis another period when the vegetation line retreated more rapidly than the


average. The period is between 1981 and 1984 and the vegetation lineappears to have retreated some 15 metres. This may be another period ofmild winters and / or more numerous seabreeze and swell activity. Thismay indicate two periods of rapid erosion in the period 1977 to 2001inclusive (25 years).

In view of this, it is suggested that the schemes to stabilise the area be testedfor two periods of rapid erosion (such as in 2001) every 25 years.


4. Option 1 – Groyne Field & Initial Beach Nourishment4.1 Description & Performance of Option 1In Rogers & Associates (1999) the Groyne Field & Initial BeachNourishment option comprised of 3 groynes and large scale initialnourishment to “saturate” the groynes. The main focus of the protectionwas from Frederick Stubbs Reserve to the coast near Tapping Way to thenorth. This scheme in the earlier report would provide little protection forthe area between Frederick Stubbs Reserve and the headland built at thecusp in 1977. At some times this section of beach could be quite narrowand recreational use would be difficult. It was thought that this should beavoided if possible.

With this in mind, a new layout of groynes was selected and is shown inFigure 4.1. There are 3 groynes equally spaced between the headland at thecusp and the beach near Tapping Way in the north. The groynes are about460 metres apart. This is about half of the length of the Southern Beachfrom the headland at the cusp to the natural and rocky headland south of theCaravan Park.

This groyne field with large scale initial nourishment was examined usingthe calibrated GENESIS model. The first model run was using thefollowing:

• the average wave conditions for 1995 to 1999,

• 3 groynes extending 20 metres beyond the initial waterline, and

• sand nourishment to form a new waterline that at the end of summerwould be 10 meters seaward of the existing waterline at the southernends of the compartments and 20 metres seaward of the existingwaterline at the northern ends of the beach compartments.

The large scale initial nourishment would provide a seaward movement ofthe waterline of 15 metres on average. This would involve about180,000 m3 of sand for nourishment spread over 1.4 km of coast. TheGENESIS model run indicated that there was a loss of about 4,000 m3 ofsand from the compartments in the year.

In view of this, the length of the groynes was increased to 25 metres fromthe existing waterline (plus approximately 15 metres across the beach givingabout 40 metres in total length). The GENESIS model was run for theaverage wave conditions, the 3 longer groynes and the 180,000 m3 of initialsand nourishment. This has been called Option 1.


The model results for this configuration and the sea conditions for a typicalyear indicated that there would still be a large seasonal flux of sand into andout of the area. There would be little, if any, loss of sand from thecompartments between the groynes. The erosion area was moved to northof the most northerly groyne near Tapping Way. In other words the groynefield had halted erosion on the Northern Beach by moving the erosionfurther to the north.

It is expected that the beach to the north of the groyne field would erodeinitially at 1 to 3 metres per year and in the longer term the erosion ratewould decrease and the average be about 1 to 2 metres per year. The setback to the new development in this area is more than 100 metres. On thisbasis, it is likely that the erosion of this beach could be tolerated for aboutone decade before additional beach stabilisation work would be needed.Such future beach stabilisation work will be included in the evaluation ofthe Net Present Value of Costs of Option 1 as an additional groyne andnourishment built about a decade after the first groyne.

Examination of the shoreline movement predicted in the GENESIS modelrun for Option 1 with average wave conditions indicated that the variousbeach compartments rotate in response to the seasonal changes in the waveconditions. In summer and autumn, the prevalence of the seabreeze activitymoves sand from south to north and the beaches between the groynes rotateanti-clockwise. In winter and spring, the westerly and northwesterly stormwaves cause movement from north to south and beaches rotate clockwise inresponse. The model suggests that for the average wave conditions theseasonal movements at the northern ends of the beach compartments wouldbe about 10 metres. At the southern ends of the compartments themovements were predicted to be up to 15 metres. The initial nourishmentwould provide a buffer of about 10 metres at the northern ends of thecompartments. At the southern ends, the buffer could be a few metres lessthan the initial 10 metres at some times. This is shown diagrammatically inFigure 4.2.

Option 1 was then examined for the mild conditions in 2001. ThisGENESIS run indicated that the beaches in the groyne field would sufferminor erosion, but the Southern Beach would lose about 46,000 m3. Inaddition, the beach to the north of the groyne field would lose about15,000 m3. The average wave conditions were then used with the GENESISmodel to examine the recovery of the Option 1 configuration. In one yearthe Southern Beach gained about 17,000 m3 and the beach compartments inthe groyne field lost a similar volume. From these results, it would appearthat Option 1 would need to be augmented with some additional


nourishment after conditions such as 2001. It has been judged that about40,000 m3 of additional nourishment may be needed.

Option 1 would halt the erosion trend and provide an additional 5 to 10metre buffer to the Northern Car Park. This buffer should be sufficient toaccommodate possible errors in the modelling and provide a buffer for somestorm erosion. The buffer is not sufficient to accommodate possiblechanges caused by Climate Change over the coming century. Option 1would transfer the erosion problem to the beaches to the north and theremay be the need for more stabilisation works in a decade or so. Such workshave been included in the financial analysis in Appendix A.

4.2 Staging Option 1 WorksThe City of Wanneroo requested that this investigation include staging thecapital works over a 3 year period to match the likely available funding.The following staged development was examined.

• Year 1 – Construct 2 groynes and complete 60,000 m3 of beachnourishment between the groynes.

• Year 2 – Construct the 3rd groyne and complete 60,000 m3 of beachnourishment.

• Year 3 – Complete 72,000 m3 of beach nourishment.

The GENESIS modelling of this staged development indicated that anadditional 12,000 m3 of beach nourishment would be needed on top of the180,000 m3 in Option 1. This is because of the losses from the groyne fieldarea during the staged development. It was concluded that staging thedevelopment over a 3 year period would be practical. The staged approachwould have the benefit of spreading the truck traffic for the groyneconstruction and beach nourishment over a number of years.

This assessment has been based on the results of the GENESIS model runs.The model is a simplification of the complex coastal processes at Quinns. Itcan be expected that the model may not be perfectly accurate in all respects.It would be wise to include a contingency sum in Year 4 to enableadjustments to the works should they become necessary to fine tune theperformance of the option.

4.3 Estimated Costs and NPV of Option 1The preliminary design of the groynes that extend 25 metres seaward of thepresent day waterline was completed using the design data in Rogers &


Associates (1999). The armour rock for the head of the groynes would beabout 5 tonnes and the crest level would need to be about 4 mAHD.

In order to minimise the impact of the groyne on the beach users, the levelof the groyne trunk could be reduced to about 2 mAHD. This would meanthat the groyne trunk would often be buried or partially buried. A long-section along the groyne is shown in Figure 4.3. Two representative crosssections of the groyne trunk and groyne head are shown in Figure 4.4

The capital cost of constructing one rock groyne that extends 25 metres pastthe existing waterline has been estimated in Table 4.1 below. The totalgroyne length is about 40 metres and the landward 15 metres would be builtat a level of about 2 mAHD.

Table 4.1 Capital Cost Estimate for Groyne

Item Quantity Rate Cost

Mobilisation & survey 1 $7,000 $7,000

Access track or pavement repair 1 $4,000 $4,000

Armour stone 1,750 m3 $30/m3 $52,500

Core stone 1,250 m3 $20/m3 $25,000

Site clean-up & demobilisation 1 $7,000 $7,000

Engineering design & mgmt 10% $9,500

Contingencies 5% $5,500

TOTAL CAPITAL COST $110,500

SAY $110,000

The cost of 60,000 m3 of sand nourishment has been estimated in Table 4.2below.


Table 4.2 Capital Cost Estimate for 60,000 m3 of Nourishment



Supply sand from local sources 4,000 m3 $6.8/m3 $27,200

Supply sand nourishment 56,000 m3 $9.4/m3 $526,400

Spread sand on site 60,000 m3 $1.5/m3 $90,000

Brush work to stop wind erosion 3,500 m2 $4/m2 $14,000





SAY $712,000

Notes: 1. Sand volumes are insitu not loose.2. Sand supply cost is $4/tonne for first 4,000 m3 then $5.5/tonne.3. Average cost of nourishment is $11.9/m3 insitu and includes brush work.

Table A.1 lists the capital and recurrent costs that are envisaged to berequired for Option 1. The capital cost has been staged over the first 3 yearsand a contingency sum in Year 4 in the following amounts.

• Year 1 - $932,000,

• Year 2 - $822,000,

• Year 3 - $857,000, and

• Year 4 - $100,000 (contingency sum).

The total capital cost is estimated to be some $2,711,000 in 2002 terms.

Following the construction, there will be the need for the following itemsthroughout the life of the option.

• Groyne maintenance about every decade,


• Episodic beach nourishment of about 40,000 m3 in years 12 and 24 torectify deficiencies caused by abnormally mild winters such as in 2001,and

• Beach stabilisation works at year 10 to address the erosion north of thegroyne field.

Table A.1 in Appendix A lists the Net Present Value of costs for Option 1for various discount rates. The analysis has been made over a 35 yearperiod and the Net Present Value of Costs is estimated to be $3,455,000 fora 6% discount rate.


5. Option 2 – Headlands & Initial Nourishment5.1 Description & Performance of Option 2The City of Wanneroo requested that as an alternative to the groynes inOption 1, rock headlands be examined. The headlands would be somewhatsimilar to the existing headland built at the cusp in 1977. The position ofthe headlands along the coast could be the same spacing as for the groynes.The headlands could be located such that the seaward side is some 25metres seaward of the present day waterline. This would put the headlandsin the same location as the head of the groynes in Option 1.

The layout for Option 2 with 3 headlands is shown on Figure 5.1 and a crosssection is shown in Figure 5.2. The initial large scale beach nourishmentwould be 180,000 m3 as for Option 1.

Option 2 was modelled using the GENESIS model. The model results forthis option indicated that there would be little if any loss of sand from thecompartments between the headlands. The erosion area was moved to northof the most northerly headland near Tapping Way. In other words theheadlands had halted erosion on the Northern Beach by moving the erosionfurther to the north.

Overall, the performance of the headlands in Option 2 was assessed to bethe same as for the groyne field in Option 1. The predicted beach rotationsare shown in Figure 5.3. The headlands would be expected to provide lessof a barrier to those using the beach as there would not be a trunk as withthe groynes. One significant disadvantage of the headlands compared togroynes is the lack of ability to make minor adjustments to the position ofthe seaward portion of the structure. With the groynes in Option 1, theycould be made slightly longer or shorter should the monitoring data indicatethat a modification was appropriate. This is a major advantage for theoptions using groynes in that they are less reliant on the coastal processesmodelling being exactly correct.

It is expected that the beach to the north of the headland field would erodeinitially at 1 to 3 metres per year and in the longer term the erosion ratewould reduce and the average be about 1 to 2 metres per year. The set backto the new development in this area is more than 100 metres. On this basis,it is likely that the erosion of this beach could be tolerated for about adecade before additional beach stabilisation work would be needed. Suchfuture beach stabilisation work needs to be included in the evaluation of theNet Present Value of Costs of Option 2.


Option 2 would halt the erosion trend and provide an additional 5 to 10metre buffer to the Northern Car Park. This buffer should be sufficient toaccommodate possible errors in the modelling and provide a buffer for somestorm erosion. The buffer is not sufficient to accommodate possiblechanges caused by Climate Change over the coming century. Option 2would transfer the erosion problem to the beaches to the north and theremay be the need for more stabilisation works in a decade or so. Such workshave been included in the financial analysis in Appendix A.

5.2 Staging Option 2 WorksThe City of Wanneroo requested that this investigation include staging thecapital works over a 3 year period to match the likely available funding.The following staged development was examined.

• Year 1 – Construct 2 headlands and complete 60,000 m3 of beachnourishment between the headlands.

• Year 2 – Construct the 3rd headland and complete 60,000 m3 of beachnourishment.

• Year 3 – Complete 72,000 m3 of beach nourishment.

The GENESIS modelling of this staged development indicated that anadditional 12,000 m3 of beach nourishment would be needed on top of the180,000 m3 needed if built all in Year 1. This is because of the losses fromthe headland field area during the staged development. It was concludedthat staging the development over a 3 year period would be practical. Thestaged approach would have the benefit of spreading the truck traffic for theheadland construction and beach nourishment over a number of years.

This assessment has been based on the results of the GENESIS model runs.The model is a simplification of the complex coastal processes at Quinns. Itcan be expected that the model may not be perfectly accurate in all respects.It would be wise to include a contingency sum in Year 4 to enableadjustments to the works should they become necessary to fine tune theperformance of the option.

5.3 Estimated Costs and NPV of Option 2The preliminary design of the headlands located with their seaward faceabout 25 metres seaward of the present day waterline was completed usingthe design data in Rogers & Associates (1999). The armour rock for theseaward face of the headlands would be about 5 tonnes and the crest levelwould need to be about 4 mAHD.


The capital cost of constructing one rock headland has been estimated inTable 5.1 below. The total length of the headland is about 30 metresbetween the heads.

Table 5.1 Capital Cost Estimate for Headland



Access road 1 $4,000 $4,000

Access bund across beach 1 $3,000 $3,000

Armour stone 2,400 m3 $30/m3 $72,000

Core stone 1,500 m3 $20/m3 $30,000





SAY $142,000

The cost of 60,000 m3 of sand nourishment has been estimated in Table 4.2to be $712,000 or $11.90/m3.

Table A.2 lists the capital and recurrent costs that are envisaged to berequired for Option 2. The capital cost has been staged over the first 3 yearsin the following amounts.

• Year 1 - $996,000,

• Year 2 - $854,000,

• Year 3 - $857,000, and

• Year 4 - $100,000 (contingency sum).




• Headland maintenance about every decade,

• Episodic beach nourishment of about 40,000 m3 in years 10 and 24 torectify deficiencies caused by abnormally mild winters such as in 2001,and

• Beach stabilisation works at year 10 to address the erosion north of thegroyne field.



6. Option 3 – Seawall & Ongoing NourishmentIn order to make an informed decision concerning the financial effectivenessof the Groyne and Headland options, the City of Wanneroo requested thatthe option to build a rock seawall and complete ongoing sand nourishmentbe examined. The latest data on coastal processes was used in theassessment of this option.

Option 3 – Seawall & Ongoing Sand Nourishment includes the followingitems:

• construction of a 400 metre long rock seawall to protect the Northern CarPark and parts of Frederick Stubbs Reserve,

• initial sand nourishment of about 80,000 m3 (insitu volume) tocompensate for the large losses experienced in 2001,

• ongoing sand nourishment of about 8,500 m3 (insitu volume) tocompensate for the typical losses due to the gradient in longshore drift,

• maintenance of the seawall from time to time, and

• episodic sand nourishment of 80,000 m3 placed over two years (40,000m3/yr) to compensate for periods of abnormal weather conditions such asin 2001.

The cost of the rock seawall was estimated using the design data fromRogers & Associates (1999) and the bathymetry from the February 2002survey. Table 6.1 below provides the details of the capital cost for theseawall.


Table 6.1 Capital Cost Estimate for Seawall



Excavation 6,000 m3 $3/m3 $18,000

Armour stone 6,100 m3 $30/m3 $183,000

Bedding layer 2,800 m3 $35/m3 $98,000

Geotextile 5,600 m2 $8/m2 $44,800





SAY $414,000

The cost of sand nourishment has been estimated to be $11.90/m3 (insituvolume) in Table 4.2 above.

Table A.3 lists the capital and recurrent costs that are envisaged to berequired for Option 3. The capital cost spent in Year 1 is as follows.

• Year 1 - $890,000, and

• Year 2 - $ 476,000.



• Annual sand nourishment of about 8,500 m3 insitu volume,

• Seawall maintenance about every decade, and

• Episodic beach nourishment of about 40,000 m3 in years 10, 11, 24 and25 to rectify deficiencies caused by abnormally mild winters such as in2001.


7. Comparison of OptionsThe Net Present Value of costs for the three options evaluated in this reportare presented in Table 7.1 below. This table also lists the Capital Cost ofthe three options.

Table 7.1 NPV & Capital Costs for Options 1, 2 and 3

Option NPV @ 4% NPV @ 6% Capital Cost

Option 1 – Groynes &Initial Nourishment

$3,738,000 $3,455,000 $2,711,000

Option 2 – Headlands &Initial Nourishment

$3,880,000 $3,585,000 $2,807,000

Option 3 – Seawall &Ongoing Nourishment

$3,937,000 $3,315,000 $1,366,000

Note: 1. Refer to Appendix A for details of discounted cash flow analysis.

This comparison indicates that all three options have quite similar NetPresent Value of costs over a 35 year period using a 4% or 6% discount rate.Certainly, the differences are less than the likely error bands in the variouscost estimates.

Option 1 has the lowest NPV when using a 4% discount rate. In addition,this option involves significant capital expenditure that can be sharedbetween the City of Wanneroo and the State Government and does not relyso much on recurrent expenditure funding.

Option 3, by comparison has the lowest NPV when using a 6% discountrate. It involves about half the amount of capital expenditure and is muchmore reliant on recurrent expenditure.

The significant ongoing sand nourishment works in Option 3 may not beacceptable to the local residents after a number of years. Manycommunities have the belief that ongoing beach nourishment is wastingmoney despite the known benefits. Consequently, the long term funding ofOption 3 may not be secure. If the recurrent funding were not available infuture years, then the performance of the scheme would be compromised.

Option 1 uses groynes to adjust the longshore transport regime and movethe erosion problem further along the coast. The use of groynes providesthe opportunity to make minor adjustments to the scheme should there bedifferences between the GENESIS model predictions and the scheme asbuilt. This is important because the real coastal processes are very complex


and the interaction of a number of factors. The modelling work is asimplification of the actual system.

Option 1 is recommended to be adopted for the long term solution to theerosion at Quinns because it has one of the lowest NPV of costs andprovides a practical scheme that can be staged over a 3 to 4 year period. Italso provides some scope for fine-tuning, should it be required.

It is recommended that the City of Wanneroo and the State Governmentreview this report, discuss the options and jointly adopt a preferred option.Then the recent erosion, this report and the preferred option should bepresented to a public meeting. At this public meeting, the acceptability ofthe use of groynes can be discussed, and hopefully the preferred option putforward by the City of Wanneroo and the State Government will besupported.

Following the public meeting, the detailed design and documentation of thepreferred option will need to be completed. The work should then be put topublic tender and constructed at the end of the coming spring to takeadvantage of the movement of sand to the south in winter and spring.


8. ReferencesDepartment for Planning & Infrastructure (2002) Quinns Rocks Plan Set

871.

Rogers & Associates, 1999. Quinns Beach Coastal Protection Works Stage1 Report. Prepared for the Shire of Wanneroo by M P Rogers andAssociates Pty Ltd, January 1999.

Rogers & Associates, 1999. Quinns Beach Coastal Protection Works Stage2 Report. Prepared for the Shire of Wanneroo by M P Rogers andAssociates Pty Ltd, October 1999.

Tremarfon Pty Ltd, 1997. Quinns Coastal Processes Study. Tremarfon PtyLtd, Perth, Western Australia, May 1997.

US Army Corp of Engineers, 1991. GENESIS: Generalized Model forSimulating Shoreline Change. Technical Report CERC-89-19. Preparedfor Department of the Army by US Army Corp of Engineers, December1989.


9. FiguresFigure 2.1 – Location Diagram 29Figure 3.1 – Vegetation Line Plot for Foreshore

Seawards of Northern Car Park 30Figure 4.1 – Option 1: Layout of Groynes 31Figure 4.2 – Option 1: Beach Rotations 32Figure 4.3 – Long Section of Groyne 33Figure 4.4 – Cross Sections of Groyne 34Figure 5.1 – Option 2: Layout of Headlands 35Figure 5.2 – Cross Section of Headland 36Figure 5.3 – Option 2: Beach Rotations 37


Figure 2.1 – Location Diagram


Figure 3.1 – Vegetation Line Plot for Foreshore Seawards ofNorthern Car Park


Figure 4.1 – Option 1: Layout of Groynes


Figure 4.2 – Option 1: Beach Rotations


Figure 4.3 – Long Section of Groyne


Figure 4.4 – Cross Sections of Groyne


Figure 5.1 – Option 2: Layout of Headlands


Figure 5.2 – Cross Section of Headland


Figure 5.3 – Option 2: Beach Rotations


10. AppendicesAppendix A Cost Analysis


Appendix A Cost Analysis

August 2002 City of Wanneroo Quinns Beach Erosion ... · M P ROGERS & ASSOCIATES Quinns Beach...

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